Elastomeric copolymers of vinylidene fluoride with other fluorinated comonomers, for example hexafluoropropylene, are well known, commercially available compositions which are resistant to chemical and thermal attack. As a result, they find particular utility as the polymeric components of o-ring seals for fuel and lubricant systems and shaft seals in automotive powertrain systems. The stability of these polymers is primarily due to their chemical structure, specifically to the high percentage of carbon-fluorine bonds which they contain. Carbon-fluorine bonds are relatively unreactive compared to bonds between carbon and other elements. Consequently, highly fluorinated vinylidene fluoride copolymers are less susceptible to chemical degradation than are non-fluorinated polymers of comparable structure.
In order to fully develop physical properties such as tensile strength, elongation, and compression set, elastomers must be cured, i.e. crosslinked. In the case of fluoroelastomers, this is generally accomplished by mixing uncured polymer (i.e. fluoroelastomer gum) with a polyfunctional curing agent and heating the resultant mixture, thereby promoting chemical reaction of the curing agent with active sites along the polymer backbone or side chains. Interchain linkages produced as a result of these chemical reactions cause formation of a crosslinked polymer composition having a three-dimensional network structure. Commonly used curing agents for fluoroelastomers include difunctional nucleophilic reactants, such as polyhydroxy compounds or diamines. Alternatively, peroxidic curing systems containing organic peroxides and unsaturated coagents, such as polyfunctional isocyanurates, may be employed. Peroxide cure systems are most often utilized to crosslink those fluoroelastomers which contain copolymerized brominated or iodinated cure site monomers or brominated or iodinated end groups derived from chain transfer agents.
The polyhydroxy and peroxide cure systems were developed and optimized for use with copolymers of vinylidene fluoride and hexafluoropropylene (VF.sub.2 /HFP copolymers). Such copolymers are the most widely used fluoroelastomers commercially. However, products made from other vinylidene fluoride-containing fluoroelastomers are sometimes preferred for use in unusually demanding environments. For example, products made from copolymers of vinylidene fluoride, tetrafluoroethylene, and propylene (VF.sub.2 /TFE/P terpolymers) are often utilized in applications wherein resistance to low temperature and basic fluids is critical. In many cases the polyhydroxy and peroxide cure processes or curing agent formulations are unsatisfactory when used to crosslink these specialty fluoroelastomers. For example, it is known to cure elastomeric VF.sub.2 /TFE/P terpolymers with either peroxide or polyhydroxy cure systems as disclosed in U.S. Pat. Nos. 4,882,390; 4,912,171; and 4,910,260. However, when such compositions are cured using a combination of polyhydroxy compound and organo-onium compound, as taught in U.S. Pat. No. 4,882,390 and 4,912,171, the cured products exhibit high compression set. The peroxide cures disclosed in U.S. Pat. No. 4,910,260 are undesirable because the curable compositions which are initially formed are extremely scorchy and would therefore be unsuitable for commercial processes.
Polyhydroxy and peroxide cures of modified VF.sub.2 /TFE/P terpolymers are also deficient. For example, unsaturated analogues of VF.sub.2 /TFE/P terpolymers can be prepared in aqueous latex or solution as disclosed in U.S. Pat. Nos. 4,759,618; 4,645,799; and 4,742,126. However, base treatment in latex is cumbersome and uniform dispersion is difficult. Solvent processes are also undesirable on a commercial scale. In addition, polyhydroxy cures of the unsaturated VF.sub.2 /TFE/P prepared by the latex route require relatively large amounts of crosslinking agent.
Because of the broad utility of elastomeric vinylidene fluoride copolymers, it would be desirable to have available a cure system which would provide a more efficient and effective cure than the polyhydroxy and peroxide processes of the prior art. It would be particularly desirable to have an improved system adaptable to such copolymers which do not contain hexafluoropropylene as well as those which contain this comonomer.